1. Technical Field
The present disclosure generally relates to a power conversion system, and more particularly, relates to a super-efficient single-stage isolated switching amplifier.
2. Description of the Related Art
A power conversion apparatus functions to convert supplied power from a power source into electrical power for a load. For an ideal power conversion process conducted by a power conversion apparatus, a DC power supply theoretically can convert essentially all supplied energy into useful energy for transmittal to a load. However, such efficiency is not always possible in a real conversion apparatus. To achieve high efficiency, a design objective is to maximally or optimally reduce or eliminate power losses resulting from certain factors in the power flow process.
Power losses incurred in a power conversion apparatus may be viewed from two perspectives, namely, the element perspective and the stage perspective. As to the element perspective, as best shown in FIG. 1, a conventional power apparatus 10 usually contains six elements which can be classified as power-lossy elements, here denoted as switching element P1, transformer element P2, rectifier element P3, amplifier element P4, filtering element P5, and auxiliary power supply element P6.
Switching element P1 usually includes one or more high frequency switches that are used to generate the right power supply needed for power conversation. Transformer element P2 usually includes one or more high-frequency switching isolation transformers. Rectifier element P3 usually includes one or more rectifier circuitries. Amplifier element P4 usually includes a switching-driven amplifier, such as a pulse width modulation (PWM) amplifier, which function to reproduce and amplify an input signal crossing a load. Filtering element P5 usually include one or more circuitries to filter out the high carrier frequency of a PWM amplifier. Auxiliary power supply element P6 usually includes an axillary power supply which in one hand consumes power, and in another hand powers components, such as modulator circuitries, driving circuitries for switching devices, and snubber circuitries absorbing ringing and spikes.
As to the stage perspective, a conventional power conversion apparatus 10, such as a conventional power amplifier 10, usually undergoes at least two stages—namely, the first stage of generating needed power supply (hereinafter referred to as “the supplier stage”) and the second stage of applying the generated power supply to amplify an input signal with one or more switching configurations (hereinafter referred to as “the amplifier stage”). At each stage, such a two-stage (or multi-stage) power amplifier 10 incurs power losses, resulting in lowering the system efficiency. Efforts have been made to combine the aforementioned two stages into single stage so as to increase system efficiency. However, such efforts have yielded only very few single-stage power conversion apparatuses 10 in the conventional art.
Regardless of the number of stage or stages, rectifier element P3, which usually includes a rectifier circuit, is usually a necessary element in a conventional power conversion apparatus 10. In the conventional art, even a single-stage power amplifier typically comprises a rectifier circuit in a power conversion process loop. On the other hand, rectifier element P3 consumes power, and is one of the largest power-loss elements among the aforementioned six elements in a conventional power conversion apparatus 10.
For example, consider: (i) a 1000-watt switching power amplifier system 15 in a half-bridge configuration, shown in FIG. 2, (ii) an amplifier element P4 driving a two-ohm load, and (iii) an amplifier stage with efficiency of 90%. In this configuration, rectifier element P3 powering the amplifier stage has an eight-amp RMS current 36 passing through the full-wave rectifier circuit 16. The power loss of each diode in the rectifier circuit 16 is about 5.6 watts, as found using the equation:Ps=½×I×Vf=0.5×8A×1.4V=5.6W                 I is the RMS DC current going through the diodes        Vf is the forward drop of the rectifier diodeAccordingly, the total power loss for the four rectifier diodes is about 22.4 watts, or about a 2.24% loss.        
Due to the power-lossy nature of rectifier element P3, efforts were expended to eliminate rectifier element P3 in switching power amplifier topologies, particularly in single-stage switching power amplifier topologies, which, as noted above are very few in the conventional art. U.S. Pat. No. 4,479,175 (hereinafter simply referred to as “the '175 patent”), titled “Phase modulated switchmode power amplifier and waveform generator” and issued to Gille et al., describes a single-stage switching power amplifier which deliberately avoids using rectifier element P3. The entire disclosure of the '175 patent is hereby incorporated by reference.
FIG. 3A illustrates such a switching power amplifier described in the '175 patent. Referring to FIG. 3A, amplifier 20 does not incorporate rectifier element P3 in the power flow loop. A first FET S1 and a second FET S2 are configured as a push-pull switching circuit to generate fixed frequency and fixed 50% duty cycle square wave to drive an isolation transformer T1. Ideally, a clean square wave 21, shown in FIG. 3B, should be present across the node A-B in amplifier 20. Power MOSFET pairs (S3, S4) and (S5, S6) act as bidirectional switches 26A and 26B, respectively. In operation, when one bidirectional switch is turned on, the other bidirectional switch is turned off. Both switches switch at the same frequency as the first FET S1 and the second FET S2. Bidirectional switches 26A and 26 are typically controlled by a phase shifted modulator (PSM).
A clock sets driving signals to drive the first FET S1 and the second FET S2. The driving signals of bidirectional switches 26A and 26B may be modulated with an input signal, shifting a phase relative to the clock signal. As the driving signal driving bidirectional switch 26A is phase-shifted by 90° relative to the clock, the corresponding reversed driving signal driving bidirectional switch 26B is phase-shifted by 270° relative to the clock, resulting in zero voltage being produced crossing load 28. As the driving signal of bidirectional switch 26A is phase-shifted by only 1° relative to the clock, the corresponding reversed driving signal driving bidirectional switch 26B is phase-shifted by 181° relative to the clock, resulting in the highest positive voltage crossing load 28.
When bidirectional switch 26A is turned on—which occurs when MOSFETs S3 and S4 as a whole is driven to act as a short circuit—bidirectional switch 26B is turned off, resulting in amplifier 20 having an equivalent circuit shown in FIG. 4A. Referring to FIG. 4B, when bidirectional switch 26B is turned off—which occurs when MOSFETs S5 and S6 are both off—MOSFETs S5 and S6 are equivalent to two capacitors connected in series, resulting in equivalent capacitance of about a few hundred pico-farads. As such, amplifier 20 is seen as an equivalent circuit shown in FIG. 4B. An equivalent circuit similar to the one shown in FIG. 4B is likewise formed when bidirectional switch 26B is turned on and bidirectional switch 26A is turned off.
It can be appreciate that the equivalent circuit of FIG. 4B is very problematic. This is because the leakage inductance of isolation transformer T1 and the aforementioned two equivalent capacitors may resonate, resulting in a large amount of excess ringing and voltage spikes cross Node A-B, as shown in the waveform of Node A-B illustrated in FIG. 4D. The large amount of ringing and spikes can cause undesirable problems. In particular, excessive voltage spikes may overstress the power MOSFETs to break them down, thus rendering amplifier 20 inoperable. Excessive ringing can produce huge EMI signals, which can also be disruptive to amplifier 20.
In the past, efforts have been made to modify amplifier 20 of the '175 patent in an attempt to reduce or the aforementioned large amount of excess ringing and voltage spikes resulting from the design of amplifier 20. For example, as shown in FIG. 5A a snubber network 32 was added to the circuit of amplifier 20 to form a switching power amplifier 30 for the objective of reducing the ringing and spikes. The snubber network 32 is made up of a capacitor C2 and a resistor R1 connected with each other in series, with the capacitance C2 being 2000 pf and the resistance R1 being 20 ohms. The resulting waveform 31 of Node A-B of amplifier 30, as shown in FIG. 5B is much clearer, when compared TO waveform 21 of Node A-B of amplifier 20. However, as clearly seen, waveform 31 still includes large voltage spikes which can be disruptive to amplifier 30 itself. Also, analysis on the power dissipated on the 20-ohm snubber resistor R1 shows that R1 consumes a non-trivial 12.7 watts of power in a 300 W switching power amplifier 30.
In fact, snubber networks, such as snubber network 32, are commonly used in a power conversion system to reduce the ringing and spike energy. However, the use of snubber networks almost always further diminishes the system efficiency, rather than improves it, since a snubber network usually reduces ringing and spikes by first absorbing the energy otherwise associated with ringing and spikes and then dissipating the absorbed energy into one or more its snubber resistors, resulting in the absorbed energy never getting reclaimed for useful (productive) purposes.
Thus, although it is appreciated in the conventional art that avoiding the use of rectifier element P3 in a single-stage switching power conversion apparatus can significantly improve system efficiency of in power conversion process, the conventional art, to Applicant's knowledge, does not have any practical solution which can, in one hand, remove otherwise disruptive “side-effects” (such as large voltage spikes) that usually comes with avoiding the use of rectifier element P3 in a single-stage configuration, and in another hand, increase the system efficiency by reclaiming energy otherwise associated with ringing and spikes for productive uses.
Accordingly, there is a need for a switching power conversion apparatus which avoids or otherwise reduces use of rectifier element P3 without abandoning a single-stage paradigm, while being able to remove or maximally reduce the aforementioned disruptive “side-effects” that usually comes with such an approach, as well as increase system efficiency by reclaiming energy otherwise associated with ringing and spikes for useful purposes.